ECU box ground.

Bewy said
It is a waste of time...& cable. Mopar inner fenders are spot welded to the f/wall, sub chassis. We are talking about 6 amps [ max], not 300-500 for the starter. There is enough 'connection' there for a more than adequate grd through the welds. Make sure the metal is clean on the box & mating surface on the fender, use a star washer between box & fender, job done.
If you were to extend this separate grd cable to the battery neg logic , then you would have grd cables from the h/lights [ about 15 amp for 4 hl car ] & one from the cigar lighter, p/windows etc. You DO need to make sure that the grd cable from engine to the body is of adequate CSA & has clean connections at each end.

I gotta disagree with you on 2 things. The first is you don't want to put the star washer between the box and the fender because you are spacing the box away from the fender the thickness of the washer thus loosing surface area to ground too. And there is no twisting action in that location. The twist of the bolt is what spins the star washer and causes it to dig in. I put the star washers on both sides one between the bolt and ecu and one on the back side of the fender between the fender and nut. You don't want to separate the ecu from the fender by putting a washer in between the two. The other thing is the ground wire it dont hurt a thing to ground it with a second ground wire. It takes 5 mins if that is a waste of time your in the wrong hobby lol.